Home battery backup, explained

Solar alone does not work in a power cut

This surprises most people, and it is worth being clear about. A grid-tied solar system shuts down automatically the moment the grid goes out, even at midday in full sun. That is a safety requirement rather than a fault: it stops your array pushing power onto lines that utility crews may be working on.

To have power during an outage you need storage and the switching hardware that goes with it, which isolates your home from the grid and then runs it from the battery and from whatever the panels are making at the time. Without that hardware, a battery on its own does not help either.

How long does it run?

Longer than people expect, provided you are not trying to run the whole house. A typical home battery holds around 13.5 kWh of usable energy. Backing up essential loads — lights, refrigeration, internet, phones, a few outlets — draws a small fraction of what a house uses flat out, which is why an essentials-only backup can last a day or more on one pack while the same battery running air conditioning might last an evening.

During a daytime outage the panels recharge the battery as it discharges, so a multi-day cut with sunshine behaves very differently from one at night. What you back up is a design decision made at the panel, and it is one of the things worth settling before you buy rather than after.

Whether it pays depends on your utility

This is the part that has changed, and it is the reason two neighbours a few miles apart can get opposite advice.

Under net billing — California’s NEM 3.0, which applies to SCE and the other investor-owned utilities — power you export earns a fraction of what you pay to buy it back in the evening. A battery closes exactly that gap: it holds the midday surplus you would otherwise have sold cheaply and gives it back at the most expensive hours. That is a real, calculable saving, and it is why storage attaches to most new solar in these territories.

Under retail net metering — LADWP, for example, which is municipally owned and outside the CPUC’s jurisdiction — the grid already banks your surplus at close to full value. A battery there buys you very little on the bill. It is a backup purchase, and an entirely reasonable one, but it should be sold to you as backup rather than as savings.

The calculator on this page identifies which utility serves your address and prices storage accordingly, so the number it shows you for a battery is the one that applies to your tariff rather than a state average.

Would a second battery help?

Sometimes, and with sharply diminishing returns. A pack can only shift what your array actually produces in surplus on an average day, and only about one full charge of it per day. Once the first battery is absorbing most of that surplus, a second one spends much of the year part-empty — it adds backup duration, which may be worth it on its own terms, but it adds much less to the bill saving than the first.

The estimator models this rather than assuming it: change the number of batteries and watch what happens to both the savings and the payback.

What this page cannot tell you

Which loads your panel can practically back up, whether your service has the capacity, where the equipment can legally and sensibly be mounted, and what your utility requires to interconnect it. Those need somebody to look at the property.

Run your address through the estimator to see what storage does to your own numbers.